1. Field of the Invention
The present invention relates to an actuator for an optical pickup which displaces a lens in order to read or write information from or to an optical disk.
2. Description of the Related Art
For the purpose of reading information from an optical disk such as a CD (compact disk) and a DVD (digital versatile disk) and recording information to the optical disk, an optical pickup has been used. The optical pickup is provided with a lens for condensing light onto an information record track of the optical disk, and is moved in a direction of the diameter of the optical disk by an external mechanism. To securely access information densely recorded on the optical disk, the optical pickup itself is also provided with a focusing mechanism which displaces the lens along an optical axis so that the lens moves close to or away from the surface of the optical disk, and an actuator which enables tracking of displacement of the lens outward or inward in the diameter direction of the optical disk.
FIG. 4 shows an enlarged schematic structure of a conventional optical pickup 1. In the optical pickup 1, a lens 2 is held so as to face an optical disk, which would be located above the lens 2 as seen in FIG. 4. The lens 2 is held by a lens holder 3, which holds a peripheral portion of the lens 2. Around the lens holder 3, a focus coil 4 is wound about an optical axis of the lens 2. A pair of track coils 5 are also wound around the lens holder 3 about an axial line parallel to long sides of the focus coil 4, which is substantially rectangular. On both sides of the pair of track coils 5, terminals 6 for applying an electric current to the focus coil 4 and the track coils 5 are disposed. Front ends of suspension wires 7, which have electrical conductivity and elasticity, are joined to the terminals 6, respectively.
The optical pickup 1 of FIG. 4 is supported in an elastic state by the suspension wires 7 between magnets 8, 9, which are plate-like permanent magnets placed so as to face each other as shown by a phantom line. The magnets 8, 9 are magnetized in a direction of plate thickness so that facing surfaces have the same magnetic pole. A magnetic circuit structured by the pair of magnets 8, 9 as described above is referred to as a unipolar face-to-face type of magnetic circuit.
FIG. 5 shows a cross sectional structure of an area around a portion which holds the lens 2 in the optical pickup 1 of FIG. 4. The lens holder 3 holds a flange portion disposed on the periphery of the lens 2. The track coil 5 has a side end in a position more outward in the diameter direction than the flange portion of the lens 2, and is wound around to a position further outward in the diameter direction.
FIG. 6A shows a direction of a magnetic flux B generated in the unipolar face-to-face type of magnetic circuit, and FIG. 6B shows a force generated at the focus coil 4. As shown in FIG. 6A, the magnets 8, 9 are placed so that magnetic poles of the same polarity face each other, and therefore, the magnetic fluxes B generated from magnetic pole surfaces repel each other and curve, spreading from the center to the outside. By placing the focus coil 4 having a substantially rectangular wound line shape in a space between the magnetic pole surfaces where the magnetic fluxes B are generated as described above, and applying an electric current thereto, a force is generated according to Fleming's left-hand rule by an electromagnetic mutual action of the electric current and the magnetic fluxes, and the focus coil 4 is driven.
It is general that the wound line shape of the focus coil 4 is basically rectangular, and substantially, only corners are curved. In the case of, for example, angularly displacing a portion corresponding to the lens holder about an axial line which is parallel to the optical axis and thereby enabling a plurality of objective lenses to switch, as disclosed in Japanese Unexamined Patent Publication JP-A 8-321062 (1996), there is a case in which track coils having a wound line shape including a lot of curved portions are used.
According to the optical pickup 1 as shown in FIG. 4, when displaced while the optical axis of the lens 2 stays perpendicular to the surface of the optical disk, the lens 2 can form an ideal spot shape on the optical disk by reducing light from a light source such as laser light, with the result that it is possible to secure a higher reading performance. In other words, it is important to displace the lens 2 while keeping the lens parallel to the surface of the optical disk. However, as to keeping the lens 2 parallel, the actuator which generates a force in the unipolar face-to-face type of magnetic circuit has problems as described below.
1) As shown in FIG. 6B, the strength and direction of the magnetic flux B passing through the track coil 5 deviates more from a direction perpendicular to the surfaces of the magnets 8, 9 in a position farther away from the center of a magnetic field. Therefore, as the track coil 5 is placed close to the center of the magnetic field, the lens 2 can be displaced in parallel to the surface of the optical disk. However, in a case where the lens holder 3 has the structure as shown in FIG. 5, the track coil 5 is blocked by the lens 2, and therefore, there is a restriction that the track coil 5 cannot be placed more inward than the lens 2 in the diameter direction. Although making the magnets 8, 9 larger with respect to the track coil 5 enables placement of the track coil 5 in a position relatively closer to the center of the magnetic field, it hinders miniaturization.
2) On the other hand, at the focus coil 4, because of the wound line shape thereof, unnecessary forces are generated by the magnetic fluxes B on both sides which do not face the magnetic pole surfaces of the magnets 8, 9, that is, on short sides in FIG. 6B. Since these unnecessary forces vary in accordance with a distance from the center of the magnetic field, when the lens 2 is displaced in a direction of placing the track coils 5, that is, a direction parallel to the long sides of the focus coil 4, the unnecessary forces generated on the short sides do not keep balance any more, and become forces tilting the lens 2 with respect to the optical axis. When the lens 2 is tilted with respect to the optical axis, it is impossible to form the aforementioned ideal spot shape, and it becomes difficult to secure a high reading performance. In order to make the force balance hard to be destroyed even though the lens 2 is moved, there is a need to use magnets 8, 9 having large magnetic pole surfaces, which hinders miniaturization.